High power radio frequency and microwave generators are necessary for the production of high power radio frequency signals. With such generators it is necessary to produce peak powers of up to at least 100MW and in this specification the term radio frequency is used to describe radiation in the high frequency, very high frequency, ultra high frequency and microwave regions of the electromagnetic spectrum. It is known that non-linear dispersive transmission lines such as that shown in FIG. 1 of the accompanying drawings, may be used to create radio frequency output pulses by modulation of an electrical impulse input which is injected into the line. Such a line can produce high peak power radio frequency signals with a peak power of more than 100MW. The line of FIG. 1 is an inductance/capacitance ladder transmission line to which non-linearity and dispersive characteristics have been added. The Figure shows five complete sections of the transmission line which could comprise as many as 100 or 200 such sections. An input pulse injected at the left hand side of the line at point 1 will propagate to the right hand side of the line to exit as an output radio frequency signal at 2. Capacitors 3 each having a value C0 and inductors 4 each having a value L0 form the primary elements of the transmission line. Saturable magnetic material is placed in each inductor 4 which causes the inductors to be non-linear. The capacitors 5 with a value C1 form coupling capacitors used capacitively to link every second cell in the line thus giving the line additional dispersive characteristics. As a result of adding non-linearity and dispersion to the transmission line an electrical pulse which propagates along the line from point 1 to point 2 is distorted according to the specific characteristics of the transmission line. Such conventional non-linear transmission lines are used as pulse modulation circuits in which for example a suitable line could modify a flat top electrical input pulse to form any suitable shape radio frequency output signal. As the injected impulse propagates along the line, energy is transferred from the injected pulse into a radio frequency signal which also propagates from 1 to 2 in the line of FIG. 2. The radio frequency signal and the remains of the injected pulse are extracted at 2.
Propagation of the injected pulse through the line of FIG. 1 provides a source of electrical energy which moves along the transmission line. The effect of including non-linearity to the line is to modify the shape of the input pulse as it propagates along the line. The propagation velocity of a particular point in the input pulse is dependent on the amplitude of the signal of that point so various parts of the signal propagate with different velocities. Under these circumstances the front of the input pulse can sharpen into a shock front with a very short rise time.
Adding dispersion to the line of FIG. 1 provides electrical characteristics which allow the generation and subsequent propagation of oscillatory signals on the line. In practice the input pulse which is injected at 1 into the line excites the formation of a radio frequency signal. Energy is transferred from the injected pulse into the radio frequency signal at the leading edge of the injected pulse. Consequently the leading edge of the injected signal is coincident with the leading oscillation in the radio frequency signal with synchronism of the signals being maintained as the input pulse and radio frequency signal propagate along the line.
At the leading edge of the input pulse energy is lost from the pulse in various ways. For example energy is lost by transfer to the radio frequency signal, by reflection from the shock front and by dissipation in the non-linear material which may be ferrite beads threaded onto lengths of metal conductor wire. The proportion of energy which is converted into radio frequency signal is dependent upon the competition between these loss processes. Additionally the proportion of energy which may be converted into a radio frequency signal is dependent upon the relative time duration of one period of the radio frequency oscillation and the time duration of the shock front which can be produced by the input pulse. In order to increase the radio frequency formation efficiency it is necessary to reduce the shock front duration in comparison with the radio frequency oscillation period.
The efficiency of circuits such as shown in FIG. 1 is limited by the duration of the shock front which can be produced by the input pulse. Such conventional circuits can form radio frequency signals with efficiencies of up to 40% at radio frequency oscillation frequencies of 1GHz. At higher oscillation frequencies the radio frequency formation efficiency decreases for the three reasons outlined above. Additional losses of radio frequency energy occur after the radio frequency oscillation has been formed. As the radio frequency oscillation propagates towards the output point 2 of the circuit, it flows through many sections or cells of the transmission line. This leads to several dissipative loss mechanisms, one of which is magnetic loss in the saturated magnetic material. Despite saturation of the magnetic material by the input pulse the high radio frequency currents which are associated with the radio frequency signal can lead to partial remagnetisation of the magnetic material. This partial remagnetisation extracts energy from the radio frequency signal and leads to an attenuation of the radio frequency signal.